Because someone in the dev team had the time to hook up their continuous integration scripts with Play Store publishing API, to the despair and jealosity of dev teams of all other apps.
This is how software should be managed. You make a change to your software, push one extra button, and in one hour all your users receive it.
"usually" is very generous. Automated testing takes effort to develop and maintain, a lot more than the rest of the CICD pipeline combined. And it's only one piece of a complete qa strategy, if it's all you have you're still using users as testers.
In case of open-source projects like Fairemail, your budget is very likely zero or in negatives. Very often it's one or few developers who make the app basically for their own daily use, and publish it on a 'use at your own risk' basis for everyone else. So yeah, if you use any open-source software, please do some testing work if you want it to improve.